docs: def preprocess(content):
      """
      对输出内容进行预处理，转为str类型 (py3)，并替换行内\r\t\n等字符为空格
      do pre-process to the content, turn it into str (for py3), and replace \r\t\n with space
      """
  
      if six.PY2:
          if not isinstance(content, unicode):
              if isinstance(content, str):
                  _content = unicode(content, encoding=sys.stdin.encoding)
              elif isinstance(content, int):
                  _content = unicode(content)
          else:
              _content = content
          assert isinstance(_content, unicode)
  
      elif six.PY3:
          _content = str(content)
  
      _content = re.sub(r'\r|\t|\n', ' ', _content)
      return _content